1. Field of the Invention
The present invention relates to a dry-sump lubrication type four-stroke cycle engine suitable for a vehicle, such as a straddle type all-terrain four-wheel vehicle or a motorcycle, and, more specifically to improvements in an oil holding and circulating system.
2. Description of the Related Art
A conventional dry-sump lubrication four-stroke cycle engine usually has an oil tank separated from a crankcase of the engine, an oil feed pump and an oil return pump (scavenging pump). The oil tank holds a predetermined quantity of an engine oil. The oil feed pump pumps up the oil from the oil tank and feeds the oil by pressure to parts needing lubrication of the engine, and the oil return pump pumps up the used oil collected in the bottom of the crankcase or an oil pan and returns the used oil into the oil tank.
This dry-sump lubrication system increases the weight, component parts and cost of the engine, and needs pipes for connecting the crankcase and the oil tank.
A dry-sump lubrication type four-stroke cycle engine previously proposed by the assignee of the present patent application in JP-A No. 288466/1994 or JP-A No. 135419/1996 is not provided with any external oil tank and has a transmission chamber formed in a rear part of a crankcase of the engine and capable of holding a predetermined quantity of the engine oil. In this previously proposed dry-sump lubrication type four-stroke cycle engine, a suction port of a scavenging pump opens into an interior of a crank chamber of the crankcase, the scavenging pump sucks up the oil collected in the crank chamber and returns the oil into the transmission chamber to maintain the interior of the crank chamber in a dry state.
An engine disclosed in JP-A No. 215411/1986 is provided with a crankcase having a crank chamber and a transmission chamber. A lower part of the transmission chamber is used as an oil chamber. The crank chamber and the transmission chamber are separated by a partition wall, and the engine oil is contained in the oil chamber in the lower part of the transmission chamber. Right and left cover chambers provided at both sides of the crankcase are connected by an oil passage. The oil is allowed to flow from the crank chamber into the cover chambers, and a scavenging pump sucks the oil from the cover chambers and discharges the oil into the oil chamber.
The engine disclosed in JP-A No. 288466/1994 or JP-A No. 135419/1996, which sucks the oil directly from the crank chamber by the scavenging pump has the following problems.
(1) The pressure in the crank chamber varies repeatedly according to the variation of the volume of the crank chamber due to the reciprocation of the piston. Since the scavenging pump sucks the oil directly from the crank chamber, the suction of the scavenging pump is affected by the pressure variation in the crank chamber, the suction rate of the scavenging pump varies and the pumping ability of the scavenging pump cannot be fully utilized.
(2) Crank arms of the rotating crankshaft contained in the crank chamber agitate and splash the oil collected in the bottom of the crank chamber. The oil exerts resistance against the revolution of the crank arms and produces agitation loss.
(3) The oil flows in the reverse direction into the crank chamber gradually while the engine is stopped for a long time.
The engine disclosed in JP-A No. 215411/1986 has the following problems.
(1) Since the right and left cover chambers always communicate with each other, and a fixed quantity of oil is held always in the covers, a large quantity of oil flows into the crank chamber when the engine is tilted beyond a certain angle and hence it is possible that the crank chamber cannot be maintained in a dry state.
(2) Since the scavenging pump sucks the oil from the cover chambers and discharges the oil into the oil chamber, air cannot be satisfactorily removed from the oil and the oil containing air is returned to the oil chamber, which causes aeration in an oil feed pump.